remove `backend` field from `AsyncQueue` #1430

Job	Run time
Release x86_64-unknown-linux-gnu (stable)	2m 33s
Test fang_derive_error (stable)	1m 27s
Test mysql (stable)	2m 54s
Clippy (stable)	1m 12s
Test postgres (stable)	3m 14s
Test blocking (stable)	3m 5s
Test sqlite (stable)	2m 28s
	16m 53s

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